In particular, the unit is designed for use in production machines equipped with a first conveyor feeding a succession of components at a first speed and at a predetermined frequency through a first station, and a second conveyor feeding a continuous succession of items at a second speed different to the first, and at the same frequency, through a second station. The unit in question is designed to transfer components from the first station to the second, so that they can be paired with the succession of items advancing on the second conveyor.
The transfer unit comprises at least one transport head capable of movement cyclically between the first and second stations in such a way as to pick up at least one component during each cycle from the first conveyor, and place it in association with a relative item advancing on the second conveyor.
The transport head is set in motion by a drive system at a speed varying in the course of each cycle between the aforementioned first speed registering at the first station and the second speed registering at the second station.
A transfer unit of the type outlined above, while suitable as a means of pairing components with items of whatever nature, finds application to advantage in the nappy/diaper manufacturing industry for the purpose of transferring such items or their components, typically adhesive tabs or layers of absorbent material, onto flat nappy/diaper blanks. It is to this type of use that reference will be made explicitly in the following specification, albeit implying no limitation in scope.
In effect, it is the practice with machines for manufacturing nappies, or diapers, to prepare a continuous strip of flat nappy/diaper blanks, unfinished, and thereafter apply finishing components to the blanks, such as elasticated tabs on either side, or crotch inserts of absorbent material, etc. . . . .
Since the components and the nappies/diapers are normally of dissimilar length, the strip of blanks and the components are supplied at different speeds to the transfer unit. Consequently, to avoid damaging both the components and the strip, and to ensure the components are not positioned wrongly on the strip, the machine must include a transfer unit equipped with transport heads such as will take up the single components at the speed with which they are caused to advance, vary their own speed during the transfer movement, and finally deposit the components on the strip at the same speed as the selfsame strip.
This effect is produced by keying the transport heads to the driven shaft of a transmission system using non-circular gears, such as will allow of converting the rotation of a drive shaft, turning at constant speed, into rotary motion of which the speed is variable periodically according to a predetermined law of motion.
In effect, depending on the profile of the gears and the way in which they are paired positionally, variable transmission ratios and different laws of motion are obtainable.
In certain instances, alternatively, the transmission may utilize cam devices that will produce the same effect as eccentric gears, that is to say a variable speed of rotation at the driven shaft, while providing a purely mechanical control over the law of motion governing the operation of the transport heads.
As an electronic alternative to mechanical cam or non-circular gear type control media, an electronic cam can be used. In other words, the speed of rotation of the transport heads is determined by the speed of rotation of an electric motor driving the transfer unit directly.
Increasing or reducing the speed of rotation of the motor, the transport heads are caused to accelerate or decelerate correspondingly.
Notwithstanding the solutions aforementioned are effective and widely adopted, they present significant drawbacks deriving principally from a complex mechanical structure, which renders the unit particularly burdensome both from the purely constructional and dimensional standpoint, and in terms of cost.
The problem becomes particularly serious when switching to a different size or shape of product, given the need to change the speed of rotation, and therefore the law of motion governing the action of the transport head.
In addition, the demands of the market are such that manufacturers will seek constantly to improve the productivity of their equipment, and this means that a higher operating speed is required, which leads in turn to greater stresses on the structural mechanical parts of the machine. In particular, a faster speed of rotation at the transfer unit dictates a greater centrifugal force on the transport heads, higher acceleration, and increased wear on mechanical components.
In an attempt to overcome these drawbacks, transfer units with two or three transport heads have been proposed, as disclosed in reference EP0743843, by way of example.
Each head is associated with a cylindrical body aligned coaxially on a fixed axis. The cylindrical bodies are mutually concentric and rotatable one independently of another, each driven by a respective motor. Each head is therefore governed by a respective law of motion.
Disadvantageously, given the particular structure and dimensions of the mechanisms by which motion is induced in each transport head, these machines can operate only with a limited number of transport heads. In effect, a number of heads greater than three would create considerable difficulties in terms of dimensions, weights and structural complexity.